Magnet system for the focusing of electron beams



March 3, 1959 w. A. vElTH ETAL 2,876,373

MAGNET- SYSTEM FOR THE FocusrNG oF ELEcTRoN BEAMS Filed Deo. 27, 1956 6Sheets-Sheet 1 March 3, 1959 w. A. vl-:ITH ET AL 2,876,373

MAGNET SYSTEM FOR THE FOCUSING OF ELECTRON BEAMS Filed Dec. v27, 1955 ssheets-sheet 2 Fig. 3

20/ N 2l 5 '18/' m 22//@ 25 /n S Z Xiamen/@f5 March 3, 1959 W, A VElTHET AL .2,876,373

MAGNET SYSTEM FOR THB FOCUSING OF ELECTRON BEAMS y Filed Deo. 27, 1956 6Sheets-Sheet 3 35 N 33 25 S 2o 27 `22 2s S N l \N s 32 36 2,9 28

s 21 31 NL 1e` N zo 26 35 2o 2l S 2 /ZS f 22 s 31 3o as 37 so Y 3e -19 vXiamen/075.

/erzerjm @t eren March 3, 1959 W. A. vElTH ET AL '2,876,373

EI.. TRON BEAMS FiledDec. 27. 1956 MAGNET SYSTEM FOR THE FOCUSING 0FELECTRON BEM/ls Filed Dec. 27, 195e March 3, 1959 w. A. vElTH ET AL 6Sheets-Sheet 6 Z Z www m V o 9 2 \1. 7 5 V IIIII/ 1 I I I I I I I l s IIl III J1 n K /l N S KS N U 9 A 4 1I I U N ,s RI|IVAOM ,l y M 3 5 5IN 2[INH l .R/u s Nm@ ZN f s 1. 5 a b 9 9. lll m.. H F F .jadezf/or. MengerJ//m C@ United States PatCfO MAGNET SYSTEM Fon THE FocUsING oF ELEcrRoNBEAMs Werner Adam Veith and Paul Meyer-er, Munich, Germany, assignors toSiemens & Halske Aktiengesellschaft, Berlin and Munich, a corporation ofGermany The present invention relates to a system of magnets forfocusing at least one electron beam, particularly for traveling wavetubes, in which the polarity of the mag- 1 netic poles or pole shoes ofthe focusing magnet, which poles or shoes are arranged one behind theother in the direction of the electro-n beam, is periodically alternatedso as to produce an alternating magnetic eld having a distribution ofthe magnetic field intensity which extends sinusoidally in the directionof the electron beam.

Upon amplifying or generating very 'short waves in traveling wave tubes,the difficulty occurs, as is known, of conducting the electron beamelo-se by the delay line in order to obtain a good coupling of theelectromagnetic wave with .the electrons kand nevertheless prevent anelectron impingement on the delay line. For this purpose it is necessarythat the electron beam be conducted in focused form along its directionof discharge.

There are already known magnetic kfocusing arrangev ments in connectionwith which either 'a lhomogeneous or an alternating magnetic field is,produ-ced along 'the direction of discharge within the dischargevessel. In order to produce a homogeneous magnetic field along thedischarge direction, magnets are arranged in star formation, eitherparallel to the direction of discharge or at the ends of the delay lineat right angles to the direction of discharge. By means of thesearrangements, a homogeneous magnetic field is produced along thedischarge path. In traveling wave tubes, this discharge path isrelatively long so that the production of a homogeneous magnetic fieldrequires a large magnetic energy ycontent of the magnets. In order tomake this field uniform, themagnet poles adjacent the direction ofdischarge are connected with soft iron cylinders. This .magnetic shuntrequires a further energy content of the magnets so'that the magnets ofthis arrangement must be 'of very large size. As a result of this thefocusing device also becomes bulky and very heavy.

For this reason, use has been made of another focusing 'pole shoes areprovided which are yso developed and arranged that a sinusoidaldistribution of the field intensity is produced along the axis of theelectron beam. This `coaxial arrangement of the magnets with respect tothe electron beam has the disadvantage that themagnets cannot be made assho-rt as desired since in such case the amplitudes of the sinusoidaldistribution of the eldjintensity along the axis of the electron bea'mwould bejtoossmall, thus limiting 'the intensity ofthe yelectronPatented Mar. 3,

current ofthe electron beam to be conducted in focused form. As a resultof this, the wavelength of the electromagnetic wave which is to beamplified or produced has a lower limit in the case of traveling wavetubes or similar veryhigh frequency tubes while there is an upper limitfor the amplification o-r the output power, as will be presentlyexplained more in detail.

Therefore, the problem of the invention was to provide a focusing devicefor traveling wave tubes or the like which assures a good focusing ofthe electron vbeam even in case of very high electron current densitiesof the electron beam.

The essential feature of the invention is that the direc,- tions ofmagnetizationin the magnet ends of the focusing magnets extendsubstantially at 'right angles through the electron beam orperpendicularly to the direction of the electron beam.

The invention makes it possible, in a particularly advantageous manner,to select the focusing magnets as long as desired in their magnetizingdirection by arranging bar magnets in a plurality of planes at rightangle to the axis of the discharge system and outside the dischargevessel in such a manner that two magnets are opposite each other in aplane symmetric to the electron beam. ln order to avoid as a result ofstray fields unnecessary increase of the flux lrequired to produce thesinusoidal field along the direction of discharge, the magnets which aredirected with the south pole facing 'the electron beam are displaced by90 with respect `to the magnets which have their north pole directedtoward the electron beam, in the plane perpendicular to the electronbeam. In order to obtain a sinusoidal field distribution along theelectron beam when the magnets are arranged at right angles thereto, thepoles of the magnets are so connectedwith the pole sho-es that thosepoles of the magnets which are adjacent to the electron beam areprovided with pole shoes shaped in such -a manner that in planes atright angles to the axis of the discharge system similar poles which areadjacent to the electron beam are connected and that the pole shoes havea polarity which alternates along the direction of discharge.

A further feature of the invention is that a plurality of focusingvmagnets, `the magnetizing directions of which extend in the magnet polesat right angles to the electron beam 'are so developed and arranged thatthe lmagnet poles extend parallel 'to each other and parallel to theelectron beam. The magnets are in such arrangement of very large widthas compared with the yknown bar magnets, this width extending over theentire discharge path, and the height and thickness of the magnets are'dimensioned so as to obtain the magnetic llux and required fieldintensity for'the sinusoidal eld necessary in the direction ofdischarge. If the field intensity requirement of such an arrangement isveryhigh, as is true in particular at Very high frequencies, itisadvantageous -to arrange two toroid-like magnets symmetrically to theelectron beam in such a manner that two similar magnet poles arealwaysopposite each other.

In accordance with Na further object and feature, the invention providesan arrangement wherein every two 'magnets are similarly spaced from theelectron beam and so located ina plane formed by the longitudinal axisland by any arbitrary straight line perpeudicularto the longitudinalaxis that their magnet poles adjacent the electron 'beam have the samepolarity. For this purpose, it is advisable from a constructionstandpoint that in 'two planes-Which are at right angles to each otherand the line of intersection of which constitutes the longitudinal axisof the electron beam, thereare always arranged two magnets in such avmanner that the .similar poles of the anagnetplane of one plane lying.in the .vicinity y,ofthe electron beam are south poles and those of theother plane are north poles.

The arrangement of the magnets parallel to the electron beam makes it ina simple manner possible to connect the south poles adjacent to theelectron beam magnetically with each other and the north poles adjacentto the electron beam magnetically with cach other by a plurality of poleshoes, such that a pole shoe which connects the south poles is followedalternately in the direction of discharge by a pole shoe which connectsthe north poles. The poles of the magnets which are located remote fromthe electron beam are suitably magnetically connected with thick softiron plates so that the magnetic impedance in these plates is negligiblysmall.

A further feature of the invention is that the focusing magnets form aneven-numbered polygon around the electron beam in a plane perpendicularthereto and that similar poles adjoin each other at the corners. Oneadvantageous structural development of this arrangement is that themagnets form a square polygon extending in' a plane perpendicular to theelectron beam, the center of which forms the axis of the electron beam.In order to produce the sinusoidal field distribution along the electronbeam, the similar poles at the corners are connected by pole shoes, thelongitudinal axis of the pole shoes extending at right angles throughthe electron beam.

It is in all arrangements according to the invention advantageous toprovide pole shoes of soft iron with a larger cross-section in thevicinity of the magnets than in the vicinity of the electron beam sothat the flux requirement necessary for the production of the sinusoidalfield distribution can be obtained from the magnets. The pole shoesserving for the magnetic coupling of similar poles are each providedwith a bore to receive the discharge vessel, for instance an elongateddischarge vessel, in the case of traveling Wave tubes. j

If means must be provided by which the sinusoidal magnetic field isinterrupted or extended by homogeneous magnetic steady fields for thecoupling or the uncoupling of electromagnetic waves, as in the case ofdelay lines of traveling wave tubes or in the case of other very highfrequency tubes, it is advantageous, for the arrangements in accordancewith the invention, for the magnets having the first and last pole shoesrespectively to be lengthened at their ends by bar magnets which aredisposed at right angles to the longitudinal axis of the dischargevessel and to provide bar magnets also located at right angles to thelongitudinal axis of the discharge vessel at a distance away from theends of the magnet necessary for the coupling and decouplingrespectively. The bar magnets may for this purpose be provided near thewall of the vessel with an annular pole shoe for connecting similarpoles. The bar magnets are suitably of such dimensions and so connectedwith soft iron plates with their poles located away from the electronbeam that a magnetic steady field is formed between the annular poleshoe and the first and last pole shoes, respectively, of themagneticvsystem in the direction of the longitudinal axis of thedischarge system.

The invention will now be explained in further detail with reference tothe drawings wherein Figs. 1a and lb show the construction anddistribution of the field intensity of the known arrangement; and

Figs. 2 to 9 show in greatly simplified and in part schematicpresentation various embodiments in the parts thereof which areessential for the invention.

Identical parts are identically referenced throughout the drawings.

In Fig. la there is shown a part of the known, previously mentionedmagnetic focusing device for traveling wave tubes in which the intensityof the field strength in zdirection alternates and is approximatelysinusoidal. Coaxial to the z-direction, which at the same timerepresents the longitudinal axis of the electron beam, annularpermananent magnets 1 are serially so arranged that similar poles areadjacent to each other. Between the similar poles, there are providedannular pole shoes 2 having cylindrical rings 3 near the dischargevessel 4. Within the discharge vessel and coaxial to the z-directionthere is arranged a helix 5 through which the electron beam isprojected. Magnetic lines of flux 6 extend from the cylindrical parts 3of the pole shoes. The direction of the magnetic lines of flux 6 dependson the polarity of the pole shoes and is shown by arrows.

In Fig. 1b, the field intensity H which occurs along the z-axis of thedischarge system of Fig. la is plotted in the direction of the z-axis.The solid sinusoidal curve 7 represents the theoretical fielddistribution which is necessaryv for the dependable focusing of theelectron beam. The dotted line 8 shows approximately the actual fielddistribution of the arrangement of Fig. 1a produced by the fact that theannular magnets 1 cannot be made magnetically so identical that theamplitudes of the sinusoidal curve of the field intensity are equal.

In order to increase the frequency to be produced or amplified in caseof a traveling wave tube shown in Fig. l, it is necessary to reduce thediameter of the helix so that the coupling with the electron beam of theelectromagnetic wave conducted over the helix is as great as possible.It is for this purpose for the same initial power or the sameamplification necessary to reduce also the cross-section of the electronbeam so that the electron current density becomes muc-h greater. Anincrease in the electron current density, however, causes the repulsionforces of the electrons to become greater so that a greaterfield-strength amplitude of the sinusoidal field strength distributionin accordance with Fig. 1b is necessary for the focused passage of theelectron beam. Since, however, the field strength of a magnet depends onits length, it is not possible in accordance with Fig. 1a to increasethe fieldA intensity if it is desired to go over to a higher frequency.Furthermore, with a higher frequency of the wave conducted over thehelix, the plasma wavelength becomes smaller. The length L of thesinusoidal wave of the field intensity, however, is in such relationshipto the plasma Wavelength that with a decrease in the plasmal wavelength,the length L of the sinusoidal wave of the field intensity must alsobecome smaller.

The arrangement in Fig. la has as a result of the two reasons explainedabove a limit wavelength of the electromagnetic wave to be amplified.

It will be seen that the greatest magnetic field occurs between thesmall spaces d of the cylindrical rings of the pole shoes 2 and that thesinusoidal field required on the z-axis represents a stray field. Thesize of this stray field depends predominantly on the values l, R and dindicated in Fig. la. l is the distance between the pole shoes 2 whichis. determinative for the stray-field shunt outside the dischargevessel; d determines the loss produced by the strong field between thecylindrical rings 3; and R is the radius of the cylindrical ring 3 onwhich the size of the field-strength distribution on the z-axis depends.It is apparent that with a decrease in l, the field strength on thez-axis also becomes less since the stray-field shunt requires a highermagnetic energy content of the magnet. With a decrease in d, the mainfield also becomes greater so that still more magnetic energy iswithdrawn from the magnetic field on the z-axis. Balancing with R wouldthen only be possible if the radius of the discharge vessel 4 (Fig. la)is made very small. However, this can be done only to the extentpermitted by precision mechanics in order that the tolerances, whichmust be very closely met in the case of traveling wave tubes, do notbecome too large. These simple considerations show that thepossibilities of the known arrangement shown in Fig. 1a are very limitedand that this arrangement cannot be used for very short electro-magneticwaves.

Fig. 2 shows schematically an arrangement of bar magnets which makes itpossible to produce the sinusoidal kfield along the z-axis of theelectron beam, in which connection the length L of one cycle of thesinusoidal magnetic field can be dimensioned in accordance with theabove conditions corresponding to the maximum frequencies occurring intraveling wave tubes. The magnets 9 and 10 are arranged horizontal andsymmetric to the electron beam, the north poles adjoining the pole shoe11. The pole shoe 11 is provided with a borehole 12 to receive thedischarge vessel such as 4 in Fig. 1a. At a right angle to thearrangement of the magnets 9 and 10, the magnets 13 and 14 are arrangedvertically and also symmetrically opposite each other with respect tothe axis z of the electron beam. In the case of this pair of magnets 13,14, in contradistinction to the first pair of magnets 9, 10, the southpoles are connected Yby the pole shoe 1S so that a magnetic field isproduced between the poles shoes 11 and 15. The pole shoe 15 also has abore-hole .12 to receive the discharge vessel such as the vessel 4 inFig. la. In the direction of the axis z of the electron beam, there thenagain follows a magnet arrangement with north poles adjoining the poleshoes.

It will be seen that the system of magnets shown in Fig. 2, incontradistinction to that shown in Fig. l, has a greater degree offreedom with respect to the dimensioning of the magnets. The length ofthe magnets, which is determinative with respect to the field strength,can be selected as large as desired. Furthermore, the arrangement of themagnets at right angles has the advantage that the. magnetic strayfields outside the field of action of the pole shoes are very small.

Fig. 3 shows the construction of the system of magnets indicated in Fig.2. The vertical magnets 13, 14 are followed by the horizontal magnets 9,10. They are then followed by a pair of vertical magnets and then againby a pair of horizontal ones, and so forth, until the entire system ofmagnets extends over the discharge path of the electron beam. Theoutside poles of the bar magnets are connected withsoft iron plates 16and 17. The soft iron plates 16 connect all similar poles to each other,while the soft iron plates 17 serve to connect the dissimilar poles. Thesoft iron plates are suitably of such size that the magnetic resistanceis negligibly small.

If one assumes that the vertical magnets 13, 14 of Fig. 3 becomeprogressively thicker in z-direction, the magnets abut against eachother without the production of the sinusoidal field distribution on thez-axis being disturbed. If the places of abutment are allowed to passinto one another, there is produced the arrangement of vertical magnetsshown in Fig. 4. The magnets 18 and 19 have a direction of magnetizationwhich is perpendicular to the z-axis. The height of these magnets issmall as compared with their width. The south poles of magnets 18 and 19are connected by trapezoidal pole shoes 20 and 21 with the rectangularpole shoe 22. The pole shoes 20, 21 and 22 are advantageously made ofsoft iron so that as high a density of the magnetic lines of forceemerging from the pole shoes 22 as desired is possible. Thecross-section 23 in the vicinity of the south pole of magnet 1S isselected larger than the cross-section 24 so that the flux requirementdetermined by the cross-section can be obtained as large as possiblefrom the magnet. The pole shoes 22 are also provided with boreholes 25to receive the discharge vessel such as 4 shown in Fig. la.

Fig. is a front View of the arrangement of the magnetic system shown inpart in Fig. 4 and Fig. 6 is a perspective view, partially in section ofthe arrangement shown in Fig. 5. The vertical magnets 18, 19, as alreadyshown in Fig. 4, are connected with pole shoes 20, 21 and 22 which arearranged one behind the other in the z-direction. Perpendicular to thispair of magnets 18 and 19 there are arranged parallel to each other twomagnets 26 and 27 which are likewise connected with pole shoes 28, 29and 30. The polarity of the' vertical pole shoe 22 must be opposed tothe polarity of the horizontal pole shoe 30. The pole shoes 22 and 30alternate in z-direction so that an alternating magnetic field isproduced. In order to obtain a purely sinusoidal distribution of thefield strength along the z-axis, the thickness of the pole shoes 22 and30 and the distance from pole shoe 22 to pole shoe 30 must be in a givenrelationship to each other. The most favorable relationship can easilybe found by experiment. As a result of the elongated magnets 18, 19, 26,27 and the alternation in zdirection of pole shoes 22 and 30, it hasbecome possible to shape the distribution of the field strength on thez-axis sinusoi-dally in the manner shown by curve 7 in Fig. lb. Anypossible places of interference in magnets 18, 19,26 and 27 arecounteracted by the elongated form. The outside poles of the elongatedmagnets 18, 19, 26 and 27 are magnetically short-circuited by means ofthe soft iron plates 31 to 38.

In Fig. 7 there is shown a variation of the arrangement of the magnetsshown in Fig. 6. This arrangement is particularly advantageous for veryhigh frequency tubes, due to the fact that the field intensity can begreatly increased by means of the toroid-like magnets 39 and 40. Asalready stated above, the field intensity of a magnet depends on itslength. Due to the toroidal shape, a possibility is afforded ofincreasing the field strength as much as desired so that this magneticsystem can be used for extremely high frequencies. The arrangement ofthe pole shoes is the same as already described in connection with Fig.6.

A'possibility of simultaneously varying the field intensity and the iiuxrequirement and therefore, for all practical purposes, the energycontent of the magnets within wide limits, is shown in Fig. 8. Themagnets 45, 46, 47 and 48 are so arranged symmetrical to the z-axis ofthe electron beam that they form the sides of a square around theelectron beam with similar poles always abutting against each other atthe corners of the square. The result is that the opposite corners ofthe square have similar polarity. The corners of similar polarity areconnected horizontally with the pole shoes 43 and 44 by way of thecentral pole shoe 30. In vertical direction, the pole shoes 41 and 42are connected by way of the central pole shoe 22.

The arrangement of the focusing magnets according to Fig. 8 represents aparticularly favorable embodiment with respect to the manner lofmanufacture and the dimensioning of the field intensity distributionobtained on the z-axis. Experimental results have given for thisembodiment extremely good approximations to the theoretical sinusoidalcurve of the field strength distribution on the z-axis.

For the coupling or uncoupling of the electromagnetic wave in the caseof very high frequency tubes it is necessary to create a magnetic steadyfield for the coupling or uncoupling space. For the arrangement inaccordance with the invention, this steady field is advantageouslyproduced at the end and at the beginning of the magnet systern. Fig. 9ashows an example of how, in the case of the arrangement shown in Fig. 6,a steady field is produced at the beginning of the magnet system.

Referring now to Fig. 9a, on the soft iron plates 34 and 38 of themagnets 18 and 19 which have the first pole shoe 22, the two bar magnets49 and 50 are arranged at right angles` to the electron beam. Thedirection of magnetization of the bar magnets 49 and 50 isadvantageously selected to be the same as the directions ofmagnetization of the magnets 18 and 19, since the length of the steadyfield is relatively large and the field intensity must be increased. Ata distance from the end of the magnet system required for the couplingor uncoupling bar magnets 51 and 52 are also arranged at a right angleto the electron beam, these magnets being 'magnetically so oriented thatthe field intensity of the steady field is further jincreased. The barmagnets S0 and 52 are com nected with a soft iron yoke 54. The barmagnets 49 and 51 are magnetically coupled in exactly the same mannerwith a soft iron yoke 53. The bar magnets 5l and 52 advantageously havean annular soft iron pole shoe 56 which together with the pole shoe 22produces a homogeneous steady tield.

In Fig. 9b, the eld intensity is plotted over the z-axis of the electronbeam. The sinusoidal curve 57 represents the magnetic field strength inthe magnet system 18, 19. Curve 57 shows the magnetic field intensitiesof the steady eld in the coupling space. This field intensity of thecoupling space is suitably selected in the order of magnitude of theeffective value of the sinusoidal wave of the field intensity of themagnet system in order to reduce the initial ripple of the electronbeam.

The present invention is not only applicable to traveling-wave tubes orthe like, but can also be employed advantageously whenever it is desiredto conduct electron beams in focused form over a relatively long path,and the term travelling wave tube as used in the claims therefore is tobe interpreted with sensible latitude as including dilerent structuresto which the invention may be applied. In addition to the embodimentsshown by way of example in the figures, the invention may also beapplied to three, four or six-wing magnet systems.

Changes may be made within the scope and spirit of the appended claims.

We claim:

1. A magnet system for focusing at least one electron beam in connectionwith a travelling wave tube and the like, comprising a plurality offocusing magnets disposed serially in a direction paralleling thedirection of propagation of the electron beam and surrounding theelectron beam for the extent of the focusing path, the directions of thelines of force extending in said magnets perpendicular to the directionof propagation of said electron beam, pole pieces respectivelycooperating with said magnets being similarly serially disposed andalternately magnetically interconnected with identical poles thereof,whereby an alternating magnetic field is produced creating along thebeam axis a substantially sinusoidal distribution of the magnetic eldintensity, said focusing magnets torming along planes extendingperpendicular to the electron beam structures exhibiting substantiallyrectangular con; iguration surrounding the electron beam, identicalpoles of said magnets being interconnected by the respective pole piecescooperating therewith.

2. A magnet system according to claim l, wherein the respective magnetpoles extend parallel to one another and parallel to the electron beam.

3. A magnet system according to claim l, wherein said magnets form astructure exhibiting a square configuration in a plane extendingperpendicular to the electron beam, said beam passing centrally throughsaid magnets.

4. A magnet system according to claim 2, wherein said magnets form astructure exhibiting a square configuration in a plane extendingperpendicular to the electron beam, said beam passing centrally throughsaid magnets.

5. A magnet system according to claim 2, wherein the longitudinal axesof said pole pieces extend in directions perpendicular to the directionof the electron beam.

6. A magnet system according to claim 3, wherein said pole pieces extendinwardly from the respective corners ot said square structure, thelongitudinal axes of said pole pieces extending in directionsperpendicular to the direction of the electron beam.

7. A magnet system according to claim 5, wherein said pole pieces aremade of soft iron exhibiting a cross-section adjacent the correspondingmagnet poles which exceeds the cross-section thereof in the neighborhoodof the electron beam.

8. A magnet system according to claim 6, wherein said pole pieces aremade of soft iron exhibiting a cross-section adjacent the correspondingmagnet poles which exceeds the cross-section thereof in the neighborhoodof the electron beam` References Cited in the tile of this patent UNTEDSTATES PATENTS 2,102,045 Thomas Dec. 14, 1937 2,157,182 Maloi May 9,1939 2,730,678 Dohler Jan. l0, 1956 2,801,361 Pierce July 30, 19572,804,548 Ruska Aug. 27, 1957 2,812,470 Cook NOV. 7, 1957 2,847,607Pierce Aug. 12, 1958

